Legion will introduce a new type of creature to World of Warcraft that might be familiar from Diablo III. It's the Treasure Goblin, or as he's known here, Treasure Demon.
People playing the alpha build of the expansion bumped into the rare fella, who looks like a little annoying demon with a huge backpack. And its abilities turned out to be the same as the Treasure Goblin's in Diablo III: It starts to cast an escape portal when players engage and activates a counter that counts down as it tries to escape.
[Pic via Bendak]
But killing it in time rewards players with some cool drops. The loot is a worthless item called "Sanctuary Coin" right now, but I wouldn't be surprised if killing a Treasure Demon in the final version will be a quick way to acquire Toys, Pets or other rare goodies.
This isn't the only thing they're implementing from Diablo III. just think about the new Transmog system and how end-game dailies will be a bit similar to Adventure Mode. All of that first appeared in Diablo.
And it also seems like Blizzard loves these greedy creatures. There's one of them hiding in Legacy of the Void as well:
Top pic: MMO-Champion